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Diffstat (limited to 'third_party/abseil-cpp/absl/base/call_once.h')
| -rw-r--r-- | third_party/abseil-cpp/absl/base/call_once.h | 226 |
1 files changed, 226 insertions, 0 deletions
diff --git a/third_party/abseil-cpp/absl/base/call_once.h b/third_party/abseil-cpp/absl/base/call_once.h new file mode 100644 index 0000000000..bc5ec93704 --- /dev/null +++ b/third_party/abseil-cpp/absl/base/call_once.h @@ -0,0 +1,226 @@ +// Copyright 2017 The Abseil Authors. +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// https://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. +// +// ----------------------------------------------------------------------------- +// File: call_once.h +// ----------------------------------------------------------------------------- +// +// This header file provides an Abseil version of `std::call_once` for invoking +// a given function at most once, across all threads. This Abseil version is +// faster than the C++11 version and incorporates the C++17 argument-passing +// fix, so that (for example) non-const references may be passed to the invoked +// function. + +#ifndef ABSL_BASE_CALL_ONCE_H_ +#define ABSL_BASE_CALL_ONCE_H_ + +#include <algorithm> +#include <atomic> +#include <cstdint> +#include <type_traits> +#include <utility> + +#include "absl/base/internal/invoke.h" +#include "absl/base/internal/low_level_scheduling.h" +#include "absl/base/internal/raw_logging.h" +#include "absl/base/internal/scheduling_mode.h" +#include "absl/base/internal/spinlock_wait.h" +#include "absl/base/macros.h" +#include "absl/base/optimization.h" +#include "absl/base/port.h" + +namespace absl { +ABSL_NAMESPACE_BEGIN + +class once_flag; + +namespace base_internal { +std::atomic<uint32_t>* ControlWord(absl::once_flag* flag); +} // namespace base_internal + +// call_once() +// +// For all invocations using a given `once_flag`, invokes a given `fn` exactly +// once across all threads. The first call to `call_once()` with a particular +// `once_flag` argument (that does not throw an exception) will run the +// specified function with the provided `args`; other calls with the same +// `once_flag` argument will not run the function, but will wait +// for the provided function to finish running (if it is still running). +// +// This mechanism provides a safe, simple, and fast mechanism for one-time +// initialization in a multi-threaded process. +// +// Example: +// +// class MyInitClass { +// public: +// ... +// mutable absl::once_flag once_; +// +// MyInitClass* init() const { +// absl::call_once(once_, &MyInitClass::Init, this); +// return ptr_; +// } +// +template <typename Callable, typename... Args> +void call_once(absl::once_flag& flag, Callable&& fn, Args&&... args); + +// once_flag +// +// Objects of this type are used to distinguish calls to `call_once()` and +// ensure the provided function is only invoked once across all threads. This +// type is not copyable or movable. However, it has a `constexpr` +// constructor, and is safe to use as a namespace-scoped global variable. +class once_flag { + public: + constexpr once_flag() : control_(0) {} + once_flag(const once_flag&) = delete; + once_flag& operator=(const once_flag&) = delete; + + private: + friend std::atomic<uint32_t>* base_internal::ControlWord(once_flag* flag); + std::atomic<uint32_t> control_; +}; + +//------------------------------------------------------------------------------ +// End of public interfaces. +// Implementation details follow. +//------------------------------------------------------------------------------ + +namespace base_internal { + +// Like call_once, but uses KERNEL_ONLY scheduling. Intended to be used to +// initialize entities used by the scheduler implementation. +template <typename Callable, typename... Args> +void LowLevelCallOnce(absl::once_flag* flag, Callable&& fn, Args&&... args); + +// Disables scheduling while on stack when scheduling mode is non-cooperative. +// No effect for cooperative scheduling modes. +class SchedulingHelper { + public: + explicit SchedulingHelper(base_internal::SchedulingMode mode) : mode_(mode) { + if (mode_ == base_internal::SCHEDULE_KERNEL_ONLY) { + guard_result_ = base_internal::SchedulingGuard::DisableRescheduling(); + } + } + + ~SchedulingHelper() { + if (mode_ == base_internal::SCHEDULE_KERNEL_ONLY) { + base_internal::SchedulingGuard::EnableRescheduling(guard_result_); + } + } + + private: + base_internal::SchedulingMode mode_; + bool guard_result_; +}; + +// Bit patterns for call_once state machine values. Internal implementation +// detail, not for use by clients. +// +// The bit patterns are arbitrarily chosen from unlikely values, to aid in +// debugging. However, kOnceInit must be 0, so that a zero-initialized +// once_flag will be valid for immediate use. +enum { + kOnceInit = 0, + kOnceRunning = 0x65C2937B, + kOnceWaiter = 0x05A308D2, + // A very small constant is chosen for kOnceDone so that it fit in a single + // compare with immediate instruction for most common ISAs. This is verified + // for x86, POWER and ARM. + kOnceDone = 221, // Random Number +}; + +template <typename Callable, typename... Args> +ABSL_ATTRIBUTE_NOINLINE +void CallOnceImpl(std::atomic<uint32_t>* control, + base_internal::SchedulingMode scheduling_mode, Callable&& fn, + Args&&... args) { +#ifndef NDEBUG + { + uint32_t old_control = control->load(std::memory_order_relaxed); + if (old_control != kOnceInit && + old_control != kOnceRunning && + old_control != kOnceWaiter && + old_control != kOnceDone) { + ABSL_RAW_LOG(FATAL, "Unexpected value for control word: 0x%lx", + static_cast<unsigned long>(old_control)); // NOLINT + } + } +#endif // NDEBUG + static const base_internal::SpinLockWaitTransition trans[] = { + {kOnceInit, kOnceRunning, true}, + {kOnceRunning, kOnceWaiter, false}, + {kOnceDone, kOnceDone, true}}; + + // Must do this before potentially modifying control word's state. + base_internal::SchedulingHelper maybe_disable_scheduling(scheduling_mode); + // Short circuit the simplest case to avoid procedure call overhead. + // The base_internal::SpinLockWait() call returns either kOnceInit or + // kOnceDone. If it returns kOnceDone, it must have loaded the control word + // with std::memory_order_acquire and seen a value of kOnceDone. + uint32_t old_control = kOnceInit; + if (control->compare_exchange_strong(old_control, kOnceRunning, + std::memory_order_relaxed) || + base_internal::SpinLockWait(control, ABSL_ARRAYSIZE(trans), trans, + scheduling_mode) == kOnceInit) { + base_internal::Invoke(std::forward<Callable>(fn), + std::forward<Args>(args)...); + // The call to SpinLockWake below is an optimization, because the waiter + // in SpinLockWait is waiting with a short timeout. The atomic load/store + // sequence is slightly faster than an atomic exchange: + // old_control = control->exchange(base_internal::kOnceDone, + // std::memory_order_release); + // We opt for a slightly faster case when there are no waiters, in spite + // of longer tail latency when there are waiters. + old_control = control->load(std::memory_order_relaxed); + control->store(base_internal::kOnceDone, std::memory_order_release); + if (old_control == base_internal::kOnceWaiter) { + base_internal::SpinLockWake(control, true); + } + } // else *control is already kOnceDone +} + +inline std::atomic<uint32_t>* ControlWord(once_flag* flag) { + return &flag->control_; +} + +template <typename Callable, typename... Args> +void LowLevelCallOnce(absl::once_flag* flag, Callable&& fn, Args&&... args) { + std::atomic<uint32_t>* once = base_internal::ControlWord(flag); + uint32_t s = once->load(std::memory_order_acquire); + if (ABSL_PREDICT_FALSE(s != base_internal::kOnceDone)) { + base_internal::CallOnceImpl(once, base_internal::SCHEDULE_KERNEL_ONLY, + std::forward<Callable>(fn), + std::forward<Args>(args)...); + } +} + +} // namespace base_internal + +template <typename Callable, typename... Args> +void call_once(absl::once_flag& flag, Callable&& fn, Args&&... args) { + std::atomic<uint32_t>* once = base_internal::ControlWord(&flag); + uint32_t s = once->load(std::memory_order_acquire); + if (ABSL_PREDICT_FALSE(s != base_internal::kOnceDone)) { + base_internal::CallOnceImpl( + once, base_internal::SCHEDULE_COOPERATIVE_AND_KERNEL, + std::forward<Callable>(fn), std::forward<Args>(args)...); + } +} + +ABSL_NAMESPACE_END +} // namespace absl + +#endif // ABSL_BASE_CALL_ONCE_H_ |